5. cardiac muscle

Question 1

what is the nature of contraction

Answer 1

constant

near synchronous in ventricles

Question 2

actions that increase the function of the heart

Answer 2

inotropy- increased force of contraction (increased SV)

chronotropy- increased frequency of contraction (increased hr)

Question 3

myosin isoforms

Answer 3

more homologous than skm, no fibre typing

Question 4

how are cardiac muscle cells adapted for synchronous electric activity

Answer 4

branched fibres are joined with intercalated discs and desmosomes
allow strong adherence and low resistance gap junctions allowing propagation of myocardial AP through heart

Question 5

intercalated discs: transverse components

Answer 5

1. desmosomes

2. fasciae adherents- antinin and vinculin

Question 6

how does a cardiac myocyte AP differ from Skm AP

Answer 6

LONG 
refractory period (influx of calcium) 
avoids tetanic contractions

Question 7

difference in T tubules

Answer 7

thick t tubules (vs thin in skeletal)
limited SR (vs extensive)
DIADS (vs triads)

Question 8

EC coupling

Answer 8

plateau results from opening of L-type calcium channels
open more slowly than Na channels (of first phase of AP)
Opening of L-type delays repolarisation
only a small amount of calcium enters cell

Question 9

where is cardiac AP propagated

Answer 9

along sarcolemma into t tubules

Question 10

L-type channels and ryanodine receptors

Answer 10

calcium enters the L-type channels which are near ryanodine receptors
this causes calcium release by SR
enables Ca to rise a little more (less than skeletal)

calcium is then requested by SR via ATPase pumps and expelled from cell by Na-Ca exchange

Question 11

transport proteins in sarcolemma

Answer 11

ATPase pump
Na-Ca exchanger
vg K channels
beta adrenergic receptor

Question 12

how does calcium transient drive cardiac contraction

Answer 12

increase in diastolic length of cardiac myocyte increases sensitivity of myocyte to calcium (Starling’s law of heart)
drugs can modulate calcium transient
L-type calcium blockers- negative inotrope- beneficial in angina as reduced contraction reduces energy demands, reducing ischaemia

Question 13

which drug lengthens calcium transient

Answer 13

digitalis/ digoxin

Question 14

cardiac muscle and ATP

Answer 14

makes lots of ATP- lots of mitochondria
70% from oxidation of fat
20% from oxidation of glucose

Question 15

cardiac muscle mitochondria

Answer 15

continuous reticulum:
Interfibrillar mitochondria
subsarcolemmal mitochondria

Question 16

cardiac muscle AP

Answer 16

1. AP arrives at opening of L-type calcium channels in plasma membrane and T tubules- Ca enters from EC fluid
2. Ca from T tubule triggers Ca release from SR due to ryanodine receptors (75-90%)
3. Ca pumped back into SR via Ca ATPase pump and expelled across plasma membrane by 3Na-Ca exchange exchanger is driven by Na gradient across membrane, maintained by Na/K ATPase

net eff

Question 17

Norepinephrine affect on cardiac muscle

Answer 17

binds to beta 1 adrenoreceptors on sarcolemme-> increase cAMP-> cAMP mediated phosphorylation of Ca channels increase Ca entry during AP, increase Ca release on SR

also activated Ca channels via G proteins
also cAMP mediated phosphorylation of phospholamban (activates ATPase- increased uptake so next AP can happen)

Question 18

regulator of cardiac muscle

Answer 18

only calcium